Mechanical models of whole muscle-tendon complexes often assume that individual muscle fiber-tendon fiber units act independently. With few exceptions, previous experimental analyses of muscle function have been based on studies of single isolated muscle fibers, activation of single motor units or activation of all the motor units of a muscle. However, under physiological conditions, muscle force is graded by recruiting variable numbers of motor units and modulating the firing rate of those motor units. The total force produced by the muscle depends not only on the intrinsic properties of the motor units but also upon mechanical interactions between motor units. In fact, significant mechanical linkages between separate motor units may arise both from links between the tendinous strands into which their fibers insert and from connective tissue links between adjacent muscle fibers. The goal of the research projects proposed in this application is to quantify the mechanical interactions between the motor units within a single muscle and to explore the mechanisms of these interactions. Two types of mechanical interactions between motor units have been previously described: (1) nonlinear summation of individual motor unit tensions, where the tension produced by concurrently activated units is greater than the linear sum of their individual tensions, and (2) "force- assistance", where the tension produced by a motor unit is enhanced following the superimposed activation and deactivation of another unit. The former effect has been attributed to a reduction of frictional forces between active and passive muscle fibers, while the latter effect may depend upon the internal length changes produced by motor unit activation. The specific aims of the proposed experiments are to further investigate the phenomena of nonlinear summation and force assistance by addressing the following questions: 1. What is the source and nature of internal friction between active and passive muscle fibers? 2. Is force assistance due to internal length changes produced by activation of motor units? 3. How are the magnitudes of nonlinear force summation and force assistance related to numbers of motor units activated, tendon compliance, motor unit type and muscle architecture?